1. Field of the Invention
This invention relates generally to antimicrobial synthetic ion exchange resins, and more particularly to synthetic ion exchange resins having antimicrobial agents encapsulated within micropores of the resins, which are used extensively in the softening of water.
2. Prior Art
Costin, U.S. Pat. No. 4,076,622, teaches a large pore macroreticular strong base anion exchange resin containing quaternary ammonium groups, tertiary sulfonium groups, quaternary phosphonium groups, alkyl pyridinium groups, or other similar moieties, for purifying aqueous solutions. The microbiocidal composition is a heavy metal halide salt of limited water solubility, such as silver chloride. The microbiocides are chemically or physically bound to the large pore macroreticular strong base anion exchange resin. Chemical binding occurs when the halide anion of the heavy metal halide salt, which is present in solution, replaces the anion originally attached to the resin and chemically bonds to the anion exchange moiety of the resin. Furthermore, the heavy metal cation of the heavy metal halide salt becomes associated with the anion, which was originally attached to the resin, and remains in the aqueous phase. Unlike chemical binding, physical binding involves the heavy metal cation of the heavy metal halide salt becoming associated with the anion, which was originally attached to the resin, and precipitating out of solution, as opposed to remaining in the aqueous phase, upon cooling and becoming physically bound within the pores located on the surface of the resin.
Hatch, U.S. Pat. No. 4,187,183 and U.S. Pat. No. 4,190,529, teach a strong base anion exchange resin containing quaternary ammonium groups, tertiary sulfonium groups, quaternary phosphonium groups, alkyl pyridinium groups, or other similar moieties, for treating high salt solutions. The bactericide is a halide ion, such as iodide or bromide, and a hypohalous acid, such as hypoiodous acid or hypobromous acid. A scavenger resin or activated charcoal must be utilized for removal or reduction of residual halogenated bactericide eluted downstream from the strong base anion exchange resin in order to purify the water meant for human consumption.
Slejko, U.S. Pat. No. 4,199,449, teaches an ion exchange column, for removing bacteria from water, having a filter with submicron pores that become clogged to indicate that the bacteria removal capacity of the column is exhausted. More specifically, the column has a large pore macroreticular strong base anion exchange resin, with or without microbiocides that may or may not be chemically or physically bound thereto, disposed within the column for removing bacteria from an aqueous influent. The column also has a submicron filter disposed downstream of the resin. Once the capacity of the resin for removing bacteria is exhausted, the submicron pores of the filter quickly become clogged with bacteria present within the aqueous effluent from the resin. Clogging causes a pressure buildup that noticeably reduces the flow of liquid exiting the filter, thereby indicating that the bacteria removal capacity of the resin within the column is exhausted.
Dillman, U.S. Pat. No. 4,382,862, teaches a cartridge, for removing impurities from water, having an elongated tube and an inlet cap separated from the elongated tube by a water permeable barrier. The elongated tube contains a large pore macroreticular quaternary ammonium anion exchange resin. The inlet cap contains a water soluble bactericide, which is calcium hypochlorite or a sodium salt of dichloroisocyanuric acid. The bactericide kills bacteria present within the incoming water prior to the water reaching the resin. Once in solution, the water soluble bactericide permeates through the barrier and also kills bacteria present in the pores and on the surface of the resin. Once the bactericide is exhausted, the resin eliminates bacteria until its capacity to eliminate bacteria is also exhausted. As a result, water recovered from the cartridge must be monitored, either continuously or at least intermittently, for the presence of bactericide or bacteria in aqueous solution.
From the aforementioned, it is readily apparent that synthetic anion exchange resins are useful for purifying water meant for human consumption. On the other hand, synthetic cation exchange resins are useful for softening water, which is accomplished by exchanging calcium and magnesium cations, present within aqueous solution, with sodium, potassium or hydrogen cations. Additionally, synthetic ion exchange resins are used in the industrial production of demineralized or deionized water.
Unfortunately however, conventional synthetic ion exchange resins utilized for softening water allow bacteria to anchor themselves onto the surface of the resin and then enter the micropores oft he resin where the bacteria are effectively shielded from disinfecting treatments. Shielding from disinfecting treatments results in the proliferation of a large number of bacteria on a conventional synthetic ion exchange bead, which is manufactured from the resin. As a result, a larger number of bacteria exist in the emerging water following ion exchange treatment than were present in the water that was introduced prior to ion exchange treatment. In addition, shielded bacteria assist in the formation of a biofilm on the bead. The biofilm creates a physical barrier between the ion exchange functional groups on the bead and the neighboring hard or unsoftened water. Therefore, water softening effectiveness of the ion exchange resin is detrimentally impaired.
The term xe2x80x9cbacteriaxe2x80x9d encompasses many bacterial strains including gram negative bacteria and gram positive bacteria. Examples of gram negative bacteria include: Acinebacter; Aeromonas; Alcaligenes; Chromobacterium; Citrobacter; Enterobacter; Escherichia; Flavobacterium; Klebsiella; Moraxella; Morganella; Plesiomonas; Proteus; Pseudomonas; Salmonella; Serratia; and Xanthomonas. Examples of gram positive bacteria include: Arthrobacter; Bacillus; Micrococcus; Mycobacteria; Sarcina; Staphylococcus; and Streptococcus. Many of the aforementioned bacterial strains, such as Acinebacter; Aeromonas; Alcaligenes; Arthrobacter; Bacillus; Chromobacterium; Flavobacterium; Micrococcus; Moraxella; Mycobacteria; Plesiomonas; Proteus; Pseudomonas; Sarcina and others, are further referred to as heterotrophic bacteria, as they are extremely hardy and can readily grow in nutrient-poor water. As a result, these heterotrophic bacterial organisms are capable of establishing large population colonies on conventional synthetic ion exchange resins. The media and the growth conditions used during the isolation of such organisms determine the nature of a Heterotrophic Plate Count (HPC). The media used to isolate HPC is a non-selective, low nutrient, solid gel matrix known as R2A agar. During incubation at 30xc2x0 C. for one week, the media allows the growth of a variety of organisms that grow in highly purified water under low nutrient environments. Heterotrophic bacteria include harmless groups of bacteria, as well as some opportunistic and disease causing bacteria, such as: Acinebacter; Aeromonas; Flavobacterium; Moraxella; Mycobacteria; Plesiomonas; and Pseudomonas.
The presence of these opportunistic, disease causing bacteria is considered to be especially harmful to the immuno-compromised and elderly populations. Because of this, HPC has been used to indicate residual chlorine disinfection and evaluate the overall quality and effectiveness of water treatment. In the United States, the Environmental Protection Agency (EPA) has a maximum allowable standard of 500 Colony Forming Units (CFU) per milliliter (ml) of water. On the other hand, Europe has a maximum allowable standard of only 100 CFU/ml.
Water softening devices using conventional synthetic ion exchange resins are known to create an HPC from about 10,000 to about 100,000 or more CFU/ml of water. A European study (Water Technology, February, 1999) found that water softening treatment resulted in a fifteen fold increase in heterotrophic bacteria and an astonishing thirty fold increase in gram negative bacteria. Because of Europe""s more stringent standards, many European Countries do not permit the use of water softeners without the utilization of further downstream disinfection.
Conventional synthetic ion exchange resins are recently being used for a newly emerging technology that is termed xe2x80x9celectrodeionizationxe2x80x9d. The technology of electrodeionization is gaining popularity because it does not use large amounts of salt, acid and caustic solutions during the regeneration of ion exchange capacity. In general, electrodeionization is a combination of mixed bed ion exchange and electrodialysis. More specifically, electrodeionization stacks are composed of mixed bed ion exchange resins packed in a specific arrangement between separate cation and anion exchange membranes. As direct current electricity is passed through the electrodeionization stacks, ions are removed from an incoming stream of water influent and transferred to a refuse stream without the use of any chemicals. After a majority of the strongly ionized components are removed, the electric current then decomposes the water effluent into its associated ions (hydrogen cation H+ and hydroxide anion OHxe2x88x92), which regenerates the resins. Because electrodeionization stacks use conventional ion exchange resins, as does water softening devices, electrodeionization stacks are likewise equally subject to harboring proliferating heterotrophic bacterial colonies and the subsequent formation of biofilm.
What is therefore needed is a method of preventing the growth of heterotrophic bacteria and the subsequent establishment of biofilm on the surface and in the micropores of the synthetic ion exchange resin. Further needed is a synthetic ion exchange resin that prevents the growth of heterotrophic bacteria and the subsequent establishment of biofilm on the surface and in the micropores of the resin. Further needed is a submicron size, high surface area, broad spectrum antimicrobial agent, which can be encapsulated within the micropores of synthetic ion exchange resin, that is nontoxic, noncarcinogenic, substantially nonleaching and durable over the lifespan of the resin.
In a broad sense, the present invention is a method of incorporating an antimicrobial agent in a synthetic ion exchange resin to protect the resin from microorganisms. More specifically, the present invention is a method of encapsulating an antimicrobial agent within a plurality of micropores of a synthetic ion exchange resin to protect the resin from the growth of heterotrophic bacteria and the subsequent establishment of a biofilm on the surface and in the micropores of the resin. Encapsulation of the antimicrobial agent is accomplished by first solubilizing the antimicrobial agent in an organic phase of a liquid monomer mixture. Second, the liquid monomer mixture is suspension polymerized. Third, the antimicrobial agent is precipitated and encapsulated within a plurality of micropores of the resin. The antimicrobial agent then exhibits migration from the micropores thereby preventing the growth of the bacteria and the establishment of the biofilm on the resin.
The method of encapsulating an antimicrobial agent within a plurality of micropores of a synthetic ion exchange resin may be applied without a substantial amount of deviation from conventional techniques of manufacturing conventional synthetic ion exchange beads from such resins. First, predetermined quantities of deaerated water, a dispersing agent, a surfactant, a catalyst, a divinylbenzene monomer, an antimicrobial agent, and a styrene monomer, or alternatively an acrylic monomer, are combined to provide a mixture. Second, the mixture is stirred at about 75xc2x0 C. to about 85xc2x0 C. for about 12 hours to about 24 hours. Third, the beads of an antimicrobial styrene-divinylbenzene copolymeric synthetic ion exchange resin, or alternatively an antimicrobial acrylic-divinylbenzene copolymeric synthetic ion exchange resin, are filtered from the mixture. Fourth, the beads are washed with water. Fifth, the beads are vacuum oven dried. Preferably, the beads are oven dried under vacuum at about 55xc2x0 C. to about 65xc2x0 C.
The present invention is also an antimicrobial synthetic ion exchange resin, which is used extensively in the softening or electrodeionization of water, that prevents the growth of heterotrophic bacteria and the subsequent establishment of biofilm on the surface and in the micropores of the resin. The antimicrobial synthetic ion exchange resin is composed of a styrene-divinylbenzene copolymer having a styrene monomer, a divinylbenzene monomer and an antimicrobial agent incorporated into the styrene-divinylbenzene copolymer. Alternatively, the antimicrobial synthetic ion exchange resin is composed of an acrylic-divinylbenzene copolymer having an acrylic monomer, a divinylbenzene monomer and an antimicrobial agent incorporated into the acrylic-divinylbenzene copolymer.
The invention also includes an electrodeionization membrane having at least one antimicrobial synthetic cation exchange resin and at least one antimicrobial synthetic anion exchange resin. The antimicrobial synthetic cation exchange resin is composed of a styrene-divinylbenzene copolymer and an antimicrobial agent incorporated therein. Alternatively, the antimicrobial synthetic cation exchange resin is composed of an acrylic-divinylbenzene copolymer and an antimicrobial agent incorporated therein. The antimicrobial synthetic anion exchange resin is composed of a styrene-divinylbenzene copolymer and an antimicrobial agent incorporated therein. Alternatively, the antimicrobial synthetic anion exchange resin is composed of an acrylic-divinylbenzene copolymer and an antimicrobial agent incorporated therein.
The present invention is also a submicron size, high surface area, broad spectrum antimicrobial agent, which can be encapsulated within the micropores of the resin, that is nontoxic, noncarcinogenic, substantially nonleaching and durable over the lifespan of the resin. Once encapsulated within the micropores of the antimicrobial resin, the antimicrobial agent exhibits migration into and through a plurality of amorphous regions within a polymeric matrix of the resin and subsequently onto the surface of the resin, until a point of equilibrium is reached. As the surface of the resin is abraded by water during softening or electrodeionization, the point of equilibrium is disrupted, which stimulates additional migration of the antimicrobial agent from the micropores to the surface of the resin until equilibrium is again reached. As a result, the antimicrobial agent protects the antimicrobial synthetic ion exchange bead from the growth of heterotrophic bacteria and the subsequent establishment of biofilm on the surface and in the micropores of the resin during water softening or electrodeionization techniques. Furthermore, the broad spectrum antimicrobial agent is an organic microbiocidal compound that is insoluble in water, substantially nonleaching and durable over the lifespan of the resin.